A two-dimensional array probe, as an ultrasonic probe, which has a plurality of transducers arranged in a matrix pattern uses an enormous number of transducers, and hence it is difficult to directly connect all the transducers to a diagnostic apparatus. For this reason, an IC (Integrated Circuit: integrated circuit chip, ASIC (Application Specific Integrated Circuit) dedicated to a transmission/reception circuit, or the like) for performing ultrasonic transmission/reception and delay addition is sometimes directly mounted on the rear surface side of each transducer.
In addition, a structure such as a substrate called an IP (Interposer) for the mounting of an IC is sometimes added to the rear surface side of each of a plurality of transducers. As a substrate (IP) for the implementation of an ASIC and connection with a transducer, a ceramic substrate made of alumina or the like is used. The use of IPs provides merits in terms of the addition of interconnections, the relocation of pads, and the like as compared with the direct mounting of ASICs on the rear surfaces of transducers.
An IP or IC is independently connected to each transducer with a solder bump, gold bump, or the like formed on the lower surface of the transducer. When using a solder bump, a plate-like transducer is tentatively mounted on an IP or IC first, and the solder is then melted by reflow. Subsequently, the melted solder is cooled to fix the transducer and the IP or the transducer and the IC to each other. When using a gold bump, a plate-like transducer coated with a conductive adhesive agent is tentatively mounted on an IP or IC first, and the resultant structure is heated in a hardening furnace to fix the transducer and the IP or the transducer and the IC to each other.
When fixing a transducer and an IP and a transducer and an IC to each other, in order to prevent the fracture of the IC due to, for example, an external force, the mounting surface of the IC, i.e., the air gap between the transducer and the IP and the air gap between the transducer and the IC, is filled with an adhesive agent called an underfill (to be referred to as a UF hereinafter) which is a liquid hardening resin in consideration of reliability. After the underfill is hardened, a cut groove is formed, extending from the upper surface of the plate-like transducer to midway in the underfill. This leads to the execution of a step of separating each transducer.
When cutting a transducer on an ASIC, there is a risk that a cut process will cause mechanical damage to the ASIC. In addition, it becomes difficult to inspect the acoustic quality of the transducer.
An IC is made of a silicon single crystal. An IP is, for example, a ceramic substrate made of an alumina ceramic material. That is, both an IC and an IP are made of materials having very high hardness. Each of these materials has a high acoustic impedance and a very low acoustic attenuation rate. For this reason, any structure cannot block acoustic propagation to the ASIC or IP. For the above reasons, when ultrasonic waves are emitted to the rear surface side of each transducer, sound waves easily propagate to the rear surface structures of the IC and IP to cause sound reverberation. This will generate a false image on an ultrasonic image.
For example, the following two methods are available as methods of reducing acoustic energy on the rear surface side of each ultrasonic transducer:
(1) providing the rear surface of a transducer with an air gap; and
(2) making a material having a very high acoustic impedance (e.g., tungsten or its carbide) tightly adhere to the rear surface of a piezoelectric transducer.
According to method (1) described above, the lower surface of each transducer is a free end, which vibrates with a ½ wavelength. According to method (2), the lower surface of each transducer is a fixed end, which vibrates with a ¼ wavelength. In either of the methods, the very large acoustic impedance difference from the rear surface side of the transducer reduces acoustic radiation to the rear surface side of the transducer. This increases the ratio of radiation onto the front surface side of the transducer, leading to an improvement in transmission sensitivity.
In the structure in method (2), since the thickness of each piezoelectric element is about half the usual thickness (½ wavelength), the electrostatic capacitance of the element increases, and the electric impedance decreases. This further improves the transmission sensitivity.
In a transducer having an air gap as in method (1) described above, however, since the acoustic impedance of air can be almost neglected as compared with a piezoelectric element, the transducer has very high performance in blocking radiation to the rear surface. However, as described above, it is very difficult to implement such a structure with the transducers of a two-dimensional array probe.
For example, there is available a method of ensuring air gaps on transducer surfaces by filling separation grooves between transducers with an adhesive agent and forming protruding shapes from the lower surfaces of the transducers. This, however, requires cumbersome steps after an element cutting step, such as adhesive agent filling, base material removal, and electrode formation. At the same time, the adhesive agent filled between the elements increases the crosstalk between the adjacent elements.
There is also available a structure in which a trench structure is formed in an effective portion of the rear surface of each transducer to support the transducer at only the two ends. However, with such a structure, it is easily imagined that when a pressure from the front surface of each transducer is applied, the element becomes susceptible to breakage, and the reliability deteriorates. In addition, a two-dimensional array probe cannot adopt this structure because each transducer has no ineffective portion.
In addition, in the case of a transducer having a rear surface layer with a high acoustic impedance that described in method (2), since the acoustic impedance ratio between the air gap structure and the flat layer of the transducer is low, acoustic radiation to the rear surface side occurs at a predetermined ratio. Furthermore, in a two-dimensional array probe, when a bump structure like that described above is used for transducer connection, since bumps themselves are a metal, the bumps become acoustic paths between the transducers and the substrates (IPs) or ICs. At the same time, acoustic waves also propagate along the underfill to cause acoustic radiation to the rear surface side of each transducer.
In the case of a two-dimensional array transducer having a structure like that described in method (2), it is necessary to greatly thin the structure on the rear surface side of the transducer. An IP needs to have a predetermined thickness because of its functional limitation, i.e., necessity to use a multilayer interconnection. For this reason, it is necessary to directly connect an IC to the transducer via a bump without using any IP. The roles of an IP are to, for example, adjust the pad position of an IC relative to a transducer, enhance the power supply performance, and serve as a control interconnection as well as facilitating IC mounting. Each IP needs to implement these roles within a corresponding IC. Although each IC can be processed to have a thickness of 100 μm or less by a polishing process, there is a risk of damaging the IC by handling (processing) at the time of IC mounting.
In addition, since a two-dimensional array transducer having a structure like that described in method (2) is based on the assumption that acoustic radiation to the rear surface side of each transducer occurs, a rear surface load member which absorbs unnecessary acoustic energy is required on the rear surface of each IC. In addition, a rear surface load member is required to have a high acoustic impedance acoustically matching with a silicon single crystal and high sound wave absorbing performance, and hence it is necessary to use a very special material. This leads to an increase in manufacturing cost. Furthermore, since it is necessary to execute an array cutting step while each transducer is mounted on a corresponding IC, there is a risk of mechanically or chemically damaging each IC.
In addition, a rubber-based material which absorbs ultrasonic waves is used for a rear-surface member of each transducer of an ultrasonic probe to attenuate ultrasonic waves. However, a rubber-based material has characteristics such as low hardness and a low glass transition point, and is morphologically unstable. That is, a rubber-based material is deformable and hence is not suitable for the basis of a structure. In addition, a rubber-based material generally has a low thermal conductivity and hence requires a process such as being kneaded with a special material, e.g., carbon fibers, to disperse heat generated from a transducer. Furthermore, when using a rubber-based material for a rear-surface member, it is necessary to add a metal or a powder such as a compound powder to the rubber-based material to bring an acoustic impedance close to a design value. For this reason, the use of a rubber-based material for a rear-surface member will lead to an increase in cost.